Piezoelectric-laser ultrasonic inspection and monitoring of thin-walled structure fabricated by directed energy deposition process based on guided waves.

Thin-walled metallic structures produced by the Directed Energy Deposition (DED) Additive Manufacturing (AM) process are prone to various fabrication defects, which hinder the wider applications of the technique in practice. In-situ inspection and monitoring methodologies are in high demand for improved quality control of printed parts. This paper presents an ultrasonic guided-wave-based method and a prototype that can potentially be used for in-situ inspection of thin-walled structures produced by DED. Lamb waves are excited by a Lead zirconate titanate (PZT) piezoelectric transducer bonded on the DED substrate remotely from the thin wall. The substrate works as a waveguide to transmit the waves which then propagate along the thin wall. A non-contact laser vibrometer is applied to measure the guide wave signals by scanning the surface of the thin wall. The mechanisms of guided wave generation and propagation along the substrate and printed part are theoretically studied. It allows for choosing proper inspection parameters to enhance the measurement sensitivity of guided waves and help interpret the signals for defect detection. Experiments were conducted with DED-produced stainless steel (316L) thin-walled structure. The new method is demonstrated in one example to detect and localize a small defect caused by inconsistent powder delivery of a fabricated thin wall sample, via analysing the B-scan ultrasonic guided wave signals. The new technique provides strong potential for in-situ online monitoring of the DED process.

Bridging optical fiber communication and underwater wireless optical communication via third harmonic generation.

In this paper, we propose an optical module, consisting of an Erbium/Ytterbium co-doped fiber amplifier (EYDFA) and a cascaded periodically poled lithium niobate (cascaded-PPLN), to bridge the conventional telecommunication and the emerging underwater wireless optical communication (UWOC). Compared with using two discrete crystals to achieve the third harmonic generation (THG), using a cascaded crystal simplifies the optical system. Under a fundamental power of 5 W at 1550 nm, we have generated an optical power of 6.54 mW at 516 nm, corresponding to a conversion efficiency of 0.1308%. Furthermore, we added a 5-km single-mode fiber (SMF) before the EYDFA, and by adjusting the seed laser power, we successfully maintained the efficiency of the THG process and the output power of the green light. Afterwards, the nonlinearity of the THG process is analyzed, and a simplified nonlinear pre-compensation method has been proposed to tailor the 4-pulse amplitude modulation (PAM4) signals. In such case, the bit error rate (BER) of the modified PAM4 (m-PAM4) can reduce by 69.3% at a data rate of 12 Gbps. Finally, we demonstrate the practicality of our proposed system by achieving a 7-m UWOC transmission in a water tank at a data rate of 13.46 Gbps in an optical dark room. This result demonstrates the feasibility of the hybrid fiber/UWOC system, highlighting its potential for practical implementation.

Superelastic NiTi Functional Components by High-Precision Laser Powder Bed Fusion Process: The Critical Roles of Energy Density and Minimal Feature Size.

Additive manufacturing (AM) was recently developed for building intricate devices in many fields. Especially for laser powder bed fusion (LPBF), its high-precision manufacturing capability and adjustable process parameters are involved in tailoring the performance of functional components. NiTi is well-known as smart material utilized widely in biomedical fields thanks to its unique superelastic and shape-memory performance. However, the properties of NiTi are extremely sensitive to material microstructure, which is mainly determined by process parameters in LPBF. In this work, we choose a unique NiTi intricate component: a robotic cannula tip, in which material superelasticity is a crucial requirement as the optimal object. First, the process window was confirmed by printing thin walls and bulk structures. Then, for optimizing parameters precisely, a Gyroid-type sheet triply periodic minimal-surface (G-TPMS) structure was proposed as the standard test sample. Finally, we verified that when the wall thickness of the G-TPMS structure is smaller than 130 µm, the optimal energy density changes from 167 J/m3 to 140 J/m3 owing to the lower cooling rate of thinner walls. To sum up, this work puts forward a novel process optimization methodology and provides the processing guidelines for intricate NiTi components by LPBF.

High-efficiency four-wave mixing in low-loss silicon photonic spiral waveguides beyond the singlemode regime.

Low-loss optical waveguides are highly desired for nonlinear photonics such as four-wave mixing (FWM), optical parametric amplification, and pulse shaping. In this work, low-loss silicon photonic spiral waveguides beyond the single-mode regime are proposed and demonstrated for realizing an enhanced FWM process. In particular, the designed 2-µm-wide silicon photonic waveguides are fabricated with standard foundry processes and have a propagation loss as low as ∼0.28 dB/cm due to the reduced light-matter interaction at the waveguide sidewalls. In the experiments, strong FWM effect is achieved with a high conversion efficiency of -8.52 dB in a 2-µm-wide and 20-cm-long silicon photonic waveguide spiral, and eight new wavelengths are generated with the pump power of ∼80 mW (corresponding to a low power density of ∼195 mW/µm2). In contrast, the FWM efficiency for the 0.45-µm-wide waveguide spiral is around -15.4 dB, which is much lower than that for the 2-µm-wide waveguide spiral. It can be seen that silicon photonics beyond the singlemode regime opens a new avenue for on-chip nonlinear photonics and will bring new opportunities for nonlinear photonic applications.

Performance analysis of an all-optical double-hop free-space optical communication system considering fiber coupling.

In an all-optical double-hop free-space optical communication system, the outage probability and bit error rate are analyzed using a composite channel. The model involves atmospheric attenuation, atmospheric turbulence, pointing error, and fiber coupling efficiency. Based on analysis of the channel model and amplifier spontaneous emission noise, the outage probability and bit error rate are obtained. For an all-optical double-hop link, fiber coupling efficiency has an important impact on outage probability, especially at relatively short total link length. By compensating for three terms of wavefront distortions, the system outage probability significantly decreases. The communication performance is further improved by optimizing the receiving aperture diameter and beam width.

High-performance silicon-graphene hybrid plasmonic waveguide photodetectors beyond 1.55 µm.

Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range. However, the reported fast graphene photodetectors mainly operate in the 1.55 µm wavelength band. In this work, we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon-graphene hybrid plasmonic waveguide, which enables efficient light absorption in graphene at 1.55 µm and beyond. When operating at 2 µm, the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz. When operating at 1.55 µm, the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz (setup-limited) and a high responsivity of ~0.4 A/W even with a low bias voltage of -0.3 V. This work paves the way for achieving high-responsivity and high-speed silicon-graphene waveguide photodetection in the near/mid-infrared ranges, which has applications in optical communications, nonlinear photonics, and on-chip sensing.

Silicon/silicon-rich nitride hybrid-core waveguide for nonlinear optics.

A silicon/silicon-rich nitride hybrid-core waveguide has been proposed and experimentally demonstrated for nonlinear applications to fill the gap between the pure silicon waveguide and the pure silicon nitride waveguide with respect to the nonlinear properties. The hybrid-core waveguide presented here leverages the advantages of the silicon and the silicon-rich nitride waveguide platforms, showing a large nonlinearity γ of 72 ± 5 W-1 m-1 for energy-efficient four-wave mixing wavelength conversion. At the same time, the drawbacks of the material platforms are dramatically mitigated, exhibiting a reduced two-photon absorption coefficient ßTPA of 0.023 cm/GW resulting in an increased nonlinear figure-of-merit as large as 21.6. A four-wave-mixing conversion efficiency as large as -5.3 dB has been achieved with the promise to be larger than 0 dB. These findings are strong arguments supporting the silicon/silicon-rich nitride hybrid-core waveguide to be used for energy-efficient nonlinear photonic applications.

Performance evaluation of continuous-wave mid-infrared wavelength conversion in silicon waveguides.

Continuous-wave (CW) mid-infrared (MIR) wavelength conversion is experimentally demonstrated using degenerate four-wave mixing (FWM) between two thulium-doped fiber (TDF) lasers in a silicon waveguide. One TDF laser is homemade with a high power and tunable wavelength, while the other one is a commercial product. The conversion efficiency is measured with respect to the pump power and the signal wavelength detuning. In the 2 µm MIR band, the measured 3 dB conversion bandwidth is 52 nm. It verifies the feasibility of FWM-based wavelength conversion based on silicon waveguides in future MIR optical communication systems.

Generation of multiphoton quantum states on silicon.

Multiphoton quantum states play a critical role in emerging quantum technologies and greatly improve our fundamental understanding of the quantum world. Integrated photonics is well recognized as an attractive technology offering great promise for the generation of photonic quantum states with high-brightness, tunability, stability, and scalability. Herein, we demonstrate the generation of multiphoton quantum states using a single-silicon nanophotonic waveguide. The detected four-photon rate reaches 0.34 Hz even with a low-pump power of 600 µW. This multiphoton quantum state is also qualified with multiphoton quantum interference, as well as quantum state tomography. For the generated four-photon states, the quantum interference visibilities are greater than 95%, and the fidelity is 0.78 ± 0.02. Furthermore, such a multiphoton quantum source is fully compatible with the on-chip processes of quantum manipulation, as well as quantum detection, which is helpful for the realization of large-scale quantum photonic integrated circuits (QPICs) and shows great potential for research in the area of multiphoton quantum science.

All-optical wavelength conversion for telecommunication mode-division multiplexing signals in integrated silicon waveguides.

An all-optical wavelength conversion method for telecommunication-band mode-division multiplexing (MDM) signals is proposed in integrated silicon waveguides by using the four-wave mixing effect with a dual-mode pump. By engineering the dispersion profile of the integrated silicon waveguide, the phase-matching conditions are realized simultaneously for the TE01 mode and the TE11 mode of the MDM signal in the telecommunication band, and they can be converted to the idler modes simultaneously. In a 1.2-cm-long waveguide, the 3-dB conversion bandwidth reaches 58 nm with a conversion efficiency of -22.3 dB pumped by a 100-mW TE01 and 105-mW TE11 pump source.

Integrated wavelength conversion for adaptively modulated WDM-OFDM signals in a silicon waveguide.

All-optical wavelength conversion for 2×11.64 GBaud adaptively-modulated orthogonal frequency division multiplexing (AM-OFDM) signals with QPSK/16QAM formats is experimentally demonstrated in a silicon waveguide. The AM-OFDM signal with partly higher- (and lower-) order formats on lower- (and higher-) frequency subcarriers has better overall conversion performance in receiving optical signal-to-noise ratio and power penalty. In comparison with the OFDM-QPSK signal, at the BER of 3.8×10-3, the bit rate increases 11.64 Gbit/s per channel almost without conversion power penalty increased by replacing the QPSK sequence with the 16QAM sequence on half subcarriers.

Experimental demonstration of nonlinear enhancement in a graphene-deposited microfiber.

Nonlinear enhancement is experimentally demonstrated by depositing graphene scraps from graphene ethanol dispersion onto a tapered microfiber. The enhancement of the nonlinearity is verified by observing the four-wave mixing (FWM) effect in the homemade graphene-deposited microfiber (GDMF). When the incident pump power is 24.2 dBm, the FWM conversion efficiency in the GDMF reaches -57.1 dB. Compared to the bare microfiber with the same dimensions, the conversion efficiency is improved by more than 3 dB. Our fabricated GDMF provides a simple way to enhance the fiber nonlinearity, and it will be suitable for nonlinear applications such as wavelength conversion and other optical signal processing operations.

Ultra-Broadband Nonlinearity Enhancement based on a Novel Graphene-Silicon Hybrid Waveguide: Structure Design and Theoretical Analysis.

A graphene-silicon hybrid waveguide with a dielectric spacer is proposed to enhance the nonlinear response in ultra-wide wavelength range by applying graphene's broadband highly nonlinear optical properties. The chemical potential of the graphene layer is tuned to satisfy the resonance condition and hence a low propagation loss is obtained. The dielectric spacer is used for avoiding additional free-carrier-absorption loss due to carrier interchange between the silicon core and the graphene layer. Aiming at the special waveguide structure with ultra-thin graphene layer, a full-vectorial theoretical model is developed to analyze its nonlinear properties. The waveguide dimensions are optimized in terms of the nonlinear parameter. The proposed hybrid waveguide exhibits high nonlinearity enhancement in an ultra-broad wavelength region covering near-infrared and mid-infrared bands. The conversion efficiency for a degenerate four-wave mixing process reaches -18.5 dB only with a pump power of 0.5 W and a waveguide length of tens of microns. In the wavelength range of 1.3-2.3 µm, the conversion efficiency can be kept stable by adopting suitable waveguide geometry and length. The corresponding 3-dB bandwidth can reach 40-110 nm for each fixed pump. The graphene-silicon hybrid waveguide has the potential to support chip-scale nonlinear applications in both near- and mid-infrared bands.

On-chip reconfigurable optical add-drop multiplexer for hybrid wavelength/mode-division-multiplexing systems.

A silicon-based on-chip reconfigurable optical add-drop multiplexer (ROADM) is presented for hybrid wavelength-division-multiplexing-mode-division-multiplexing systems. The present ROADM consists of a four-channel mode demultiplexer, four wavelength-selective thermo-optic switches based on microring resonators, and a four-channel mode multiplexer. With the present ROADM, one can add/drop one of wavelength channels of any mode to/from the multimode bus waveguide successfully with an excess loss of 2-5 dB and an extinction ratio of ∼20 dB over a wavelength range of 1525-1555 nm.

Directly modulated green-light diode-pumped solid-state laser for underwater wireless optical communication.

It is widely known that a diode-pumped solid-state laser (DPSSL) has very limited modulation bandwidth. Recently, we directed our attention toward the opportunities for directly modulating a DPSSL to generate high-speed green-light signals, with high power and superior beam quality, which are highly desirable in underwater wireless optical communication. The constraint imposed by the limited modulation bandwidth of a DPSSL is circumvented with the strategy of orthogonal frequency-division multiplexing and power loading. With a compact DPSSL dismantled from a low-cost laser pointer, we achieve net bit rates of 108.55 Mb/s for the 64 quadrature amplitude modulation (QAM) signal at a bit error rate (BER) of 6.42×10-4 and 89.55 Mb/s for the 32 QAM signal at a BER of 4.81×10-4, respectively, over a 2 m underwater channel. When the underwater transmission distance is increased to 6 m, the BERs are still below the forward error correction (FEC) limit of 3.8×10-3.

Broadband mid-infrared fiber optical parametric oscillator based on a three-hole suspended-core chalcogenide fiber.

A mid-infrared fiber optical parametric oscillator is proposed and designed based on a three-hole As(2)S(5) suspended-core fiber (SCF). The eigenmodes of the SCF are depicted and the pump condition for single-mode operation is analyzed. The zero-dispersion wavelength is shifted to 2 µm by tuning the core diameter of the SCF. Using the degenerate four-wave mixing coupled-wave equations, a tuning range of the idler wavelength from 2 to 5 µm and a maximum conversion efficiency of 19% are numerically predicted in a 0.1-m-long SCF pumped by a 2.7 W thulium-doped fiber laser.

[Authentication of Age of Bloodstains Using UV Visible Reflection Spectrum].

The age of bloodstains is tightly related to the time elapsed since the crime was committed. The inference of the time that the crime was committed is of great significance to solve the case, and it was also a difficult problem in judicial authentication. Therefore, establishing a method of rapid determination of bleeding time is very necessary. Using a UV-visible spectrometer with a reflection accessory called ISR-240A and whiteboard as a reference standard, the reflection spectra of blood gauze, aluminum, glass and plastic were measured every hour under the condition of 16 °C and 70% humidity within 8 hours. Using SPSS to process the data, R541/R577 was figured out and linear fitting was completed. The same method was carried on on the gauze of blood at 24 °C. The results showed that, within 8 hours, spectral reflectance values of blood at 541 and 577 nm increased gradually with the passage of injury time. In addition to the R2 of the glass with blood is 0.769, the rest of the R2 were greater than 0.900. The values of F were greater than F0.05 (1, 6) = 5.59. Therefore, the linear regression model is significant meaningful. The method of using a UV-Visible spectrometer without doing any operations of the test samples is simple. Moreover, it does no harm to the further inspection in aspects of personal information, which is suitable for the judicial practice.

[Impacts of climate change on food production in Gansu: A review].

The climate of Gansu turned to be overall warming-drying and partly warming-wetting since 1986. In contrast to that of 1960, the average annual temperature had raised by 1.1°C with the average annual precipitation decreased by 28 mm correspondingly, which made the arid region expanded southward by 50 km in 2010. Climate warming increased the growth period effective accumulated temperature of main food grain crops and lengthened the crop growth period. It changed crop maturity, crop disposition, cropping system and generally increased the cultivatable area and planting altitude above the sea level of major crops and expanded northward the multiple cropping system, which further resulted in expansion of autumn grain crop sown area, shrink of summer grain crop sown area, and replacement of strong winter early maturing varieties by weak winter middle late maturing varieties. It benefited the crop yield by increasing the use efficiency of photo-thermal resources. Warming-wetting climate increased the climate productivity of oasis crop while warming-drying weather decreased the climate productivity of rainfed crops, which were mostly determined by the precipitation regimes and water conditions. Any advanced technique that can increase precipitation use ratio and water use efficiency as well as improve and promote soil quality and fertility should be regarded as an effective countermeasure to increase food grain production under climate change in Gsansu. So, selecting and breeding new crop varieties with the characteristics of strong resistance, weak winter, middle-late mature and high water use efficiency, establishing new planting structure and cropping system that suitable to the precipitation and temperature features of changed climate, are the development direction of food grain production in Gansu to cope with the climate change.

Reconfigurable dual-channel all-optical logic gate in a silicon waveguide using polarization encoding.

A reconfigurable dual-channel all-optical logic gate is proposed and experimentally demonstrated using four-wave mixing in a silicon waveguide for polarization encoding signals. Six logic functions, XNOR, AND, NOR, XOR, AB¯, and A¯B are implemented at two different wavelength channels by adjusting the polarization states of two 10 Gb/s non-return-to-zero polarization-shift keying (NRZ-PolSK) signals modulated by 10-bit on-off keying (OOK) sequences. The eye diagrams of the logic signals are clearly observed, and the logic functions are well demonstrated as the two incident NRZ-PolSK signals are both modulated by the OOK sequences, which originate from 2(31)-1 pseudo-random binary sequences.

Low-power-penalty wavelength multicasting for 36 Gbit/s 16-QAM coherent optical signals in a silicon waveguide.

All-optical wavelength multicasting has been experimentally demonstrated for 36 Gbit/s 16-quadrature amplitude modulation signals based on four-wave mixing processes in a silicon waveguide with multiple pumps. In our experiment, dual pumps are injected together with the signal into the waveguide and nine idlers are generated, involving five wavelength multicasting channels. Coherent detection and advanced digital signal processing are employed, and the recovered constellation diagrams of the multicasting idlers show a root-mean-square error vector magnitude degradation as small as 2.74%. The bit error rate (BER) results are measured for these multicasting idlers, and the power penalties are all lower than 0.96 dB at the BER of 3.8×10(-3) (corresponding to the forward error correction threshold).